1. Field of the Invention
The present invention relates in general to wireless telecommunications networks and more particularly to a method for generating neighbor lists for handoffs between the various wireless coverage areas of a wireless telecommunications network.
2. Description of Related Art
In wireless telecommunications networks, mobile stations are typically able to transmit and receive voice, data, or other media in a given format over an air interface. In North America, the most commonly used formats are the Advanced Mobile Phone System (AMPS), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). Typical TDMA formats are specified in interim standards IS-54B and IS-136, published by the Telecommunications Industries Association/Electronic Industries Association (TIA/EIA). These IS-54B and IS-136 interim standards are fully incorporated herein by reference. A typical CDMA format is specified in the ANSI/TIA/EIA-95-B-99 (“ANSI-95”) standard, which standard is fully incorporated herein by reference.
Whether the wireless telecommunications network uses AMPS, TDMA, CDMA, or some other format, the network provides a plurality of wireless coverage areas, which are typically defined by a plurality of base stations. In particular, each base station defines a geographic area, termed a “cell,” in which mobile stations can engage in wireless communication with that base station. A base station may also use directional antennas to divide its cell into smaller geographic areas, termed “sectors.” In addition to their geographic locations, the cells or sectors are typically distinguishable based on the different control channels, or pilot channels, that the base stations transmit into them.
Each base station is typically connected, either directly or indirectly with a mobile switching center (MSC). The MSCs typically control the operation of a number of base stations and typically switch calls between these base stations and the public switched telephone network (PSTN).
When a mobile station currently in wireless communication with a given base station starts to move out of the range of good wireless communication during the course of a call, the wireless network will attempt to find another base station, or another sector of the given base station, with which the mobile station can continue wireless communication with minimal interruption. This process of transferring a mobile station from one cell to another, or from one sector to another, is termed “handoff.”
Though the details of the handoff process may differ, depending on whether the wireless network uses AMPS, TDMA, CDMA, or some other format, in each case the network will typically have defined for each cell or sector a set or list of “neighbor” cells or sectors that serve as handoff candidates. For example, in AMPS systems, the base station currently communicating with a given mobile station will monitor the mobile station's signal strength, and, if the signal strength falls below a threshold value, the base station informs the MSC. The MSC then orders the base stations of “neighboring” cells to monitor the signal from mobile station and to report back the results. When the MSC receives the results, it will typically initiate a handoff to the cell that reported the highest signal strength. Notably, requiring “neighboring”base stations to measure signal strengths and to report the results to the MSC takes up the cellular system's resources. For this reason, it is desirable to minimize the number of cells or sectors in a given “neighbor set” or “neighbor list.”
TDMA differs from AMPS in that it typically uses mobile assisted handoff (MAHO). In the MAHO approach, each mobile station periodically monitors the signal strength of the control channel of its current cell or sector, as well the signal strengths of the control channels of the cells or sectors in its “neighbor list.” Typically, the wireless network transmits to the mobile station a “neighbor list” to use each time it enters a new cell or sector. The mobile station periodically reports its signal strength measurements for the various members of its neighbor list to its current base station, and the cellular system typically initiates handoffs based on these measurements. Because of the limited number of time slots available to each mobile station with which to make signal strength measurements and to report the results, it is desirable to keep the size of each neighbor list to a minimum. Moreover, the IS-54B TDMA specification sets a limit of 12 cells or sectors in each neighbor list, and the IS-136 TDMA specification sets a limit of 24 cells or sectors in each neighbor list.
CDMA systems take advantage of a CDMA mobile station's ability to communicate on more than one channel at a time to perform, to the extent possible, “soft” handoffs, which are unavailable in TDMA or AMPS systems. During a “soft” handoff, a mobile station already in communication with a first cell or sector, begins to communicate with a second cell or sector, while continuing to communicate with the first cell or sector. To determine when a soft handoff is available, CDMA mobile stations periodically measure the signal strengths of a plurality of pilot channels, with each pilot channel corresponding to a cell or sector, and report at least some of the measured signal strengths to the cellular system. More particularly, CDMA mobile stations typically maintain at least three lists of pilot channels: an active set, a candidate set, and a neighbor set. The active set consists of the pilot channels with which the mobile station is currently communicating. The candidate set consists of the pilot channels that the mobile station is receiving with signal strengths above a threshold value, indicating that good communication is possible with the corresponding cell or sector. The neighbor set consists of pilot channels of “neighboring” cells or sectors that are deemed to be potential candidates for handoff.
In operation, the CDMA network transmits the neighbor set to the mobile station, and the mobile station periodically scans the pilot channels in the neighbor set to measure their signal strengths. When the mobile station encounters a pilot channel with a signal strength that exceeds the threshold value, the mobile station adds it to the candidate set. Typically, the mobile station then requests the cellular system to allow communication with the corresponding cell or sector. If such communication is granted, the mobile station adds the pilot channel to the active set. If the signal strength of a pilot in the active set or candidate set falls below a threshold, it may be removed from the candidate set and returned to the neighbor set.
In this way, the neighbor set defines the sectors or cells with which the mobile station may potentially effect a soft handoff. However, because the process of scanning the pilot channels in the neighbor set consumes part of the mobile station's bandwidth, the size of the neighbor set should be minimized. In fact, the ANSI-95 standard limits the number of pilot channels that can be included in the neighbor list to ten. The ANSI-95 standard also allows the pilot channels in the neighbor set to be assigned one of three priority levels: 2, 1, or 0. The priority level determines how frequently the mobile station scans for that pilot channel, with level 2 being the highest priority and level 0 being the lowest priority. Thus, for efficient operation, the neighbor set must be properly defined and each pilot channel in the neighbor set must be properly prioritized.
Whether a wireless network uses AMPS, TDMA, or CDMA technology, proper definition of the “neighbor set” or “neighbor list” for each cell or sector is crucial. If the neighbor set is too large, then the cellular system will consume resources unnecessarily. However, if the neighbor set is too small, then the cellular system may miss handoff opportunities, with the result that calls may be dropped. Moreover, in the case of CDMA systems, the pilot channels in each neighbor set should also be prioritized to achieve the most efficient operation.
Several methods for defining the “neighbors” of each cell or sector are known in the art. To a first approximation, the “neighbors” of a given cell or sector may be taken to be the cells or sectors that are closest geographically. Neighbor sets defined in this way will not, in general, be optimal. This is because the geographical approach does not take into account differences in the power transmitted by the different base stations and does not take into account the effects of buildings and topography.
As a result, neighbor sets are usually determined empirically. For example, cellular system operators may travel through the network's wireless coverage areas with receivers to measure, in each cell or sector, the signal strengths of the control or pilot channels of the other cells or sectors. The neighbor set for a given cell or sector is then defined as the set of other cells or sectors whose signal strengths were highest in the given cell or sector. This procedure is very time consuming, however, particularly since it may need to be re-done when changes are made to the wireless network or to the associated topography. The prior art also discloses ways of automating this measurement process by having the cellular system keep track of the measured signal strengths, as reported by the mobile stations during normal operation. However, maintaining this amount of information on a daily basis is undesirable because it consumes a great deal of system resources.
In addition to the above difficulties, the present inventors have discovered that the approach of defining neighbors for handoff purposes based entirely on signal strength is fundamentally flawed because it does not take usage into account. For example, as between two handoff candidate cells that have about equal signal strength, the cell with the lower traffic level is a more desirable handoff candidate from a system perspective. In fact, a cell with a very high traffic level may simply be unavailable for handoff.
Accordingly, it is desirable to provide more efficient methods for generating neighbor lists or for tuning existing neighbor lists.